So in Peace Talks it sounds as though the Swords *may* have an Angel in them, or at the very least, loaded with angelic power. Angels can fall, and become dark. I'm wondering if the events in Mirror Mirror will be in part that where Harry's Choice led to Mavra using the Sword to kill an innocent, to force it to spill innocent blood. The result is that the Sword falls, and becomes a weapon of darkness. Instead of leveling the playing field to slay the enemy it does the opposite and empowers the person looking to fight good.
It would be interesting to see the Sword instead of bright light it's shrouded in a dark mist.